1. Field of the Invention
The present invention relates to optical waveguides which merge together and have a branch angle within a specific range.
2. Description of the Related Art
Optical communication systems using fiber optical transmission lines are being used to transmit relatively large amounts of information. However, as users require larger amounts of information to be rapidly transmitted, and as more users are connected to the systems, a further increase in the transmission capacity of optical communication systems is required.
Optical waveguides are being used for this purpose. For example, optical waveguides are being used in optical external modulators to increase modulation rate, and in optical wave filters for wavelength-multiplex communications, to thereby increase transmission capacity of optical communication systems.
Optical waveguides are also used in various types of optical devices for taking measurements.
For such uses of optical waveguides, it is desirable to form optical waveguides in an integrated circuit (typically referred to as a "chip"). Unfortunately, conventional optical waveguides typically have required lengths which are so long that they prevent desired functions from being implemented within a single chip. For example, optical waveguides may have a required length as long as several centimeters. This makes it difficult to implement optical waveguides in a single chip, despite optical waveguide widths as narrow as several micrometers to several tens of micrometers.
In order to circumvent this problem, optical waveguides can be"folded" many times by using waveguide reflectors so as to implement a long length optical waveguide within the confines of a single chip.
For example, FIG. 1 is a diagram illustrating a conventional optical waveguide having a folded waveguide structure and formed on a single chip as a Mach-Zehnder modulator. (This device can be found, for example, in Institute of Electronics, Information, and Communication Engineers, Electronics Society Conference, C-151, 1995, which is incorporated herein by reference).
Referring now to FIG. 1, waveguides 100 make a U turn at one end of the chip via a folded waveguide portion 101. A reflection-type wave plate 102 is provided where light is reflected. Through a reflection, TE light changes to TM light, and TM light changes to TE light, thereby achieving a modulator which does not discriminate polarization.
In this example, the waveguides are folded in a geometrical manner (folding angle: 9 degrees). Such a configuration has problems in device performance. Namely, when such a simple configuration is employed, a length of a waveguide where light beams meet is rather short. Even when a reflection surface is formed by cutting saw or the like, a displacement as small as 10 .mu.m may cause a serious deviation from the reflection geometry, thereby creating a large loss. In this example, a loss amounting to 2 dB may be suffered.
When the folding angle is decreased so as to be as small as several degrees, reflected light returns back to a waveguide where the original light came through. This is presents many problems.
In view of the above described problems, a configuration using folded waveguides has never been used in practice.
FIG. 2 is a diagram illustrating a conventional wavelength-filter-insertion type device. (This device can be found in Institute of Electronics, Information, and Communication Engineers, Electronics Society Conference, C-229, 1995, which is incorporated herein by reference.)
Referring now to FIG. 2, the device includes waveguides 110, a 1.55 .mu.m port 112, a common port 114, a dielectric multi-layer filter 116 and a quartz-family optical waveguide 118 formed on a Si substrate 120. Waveguides 110 are arranged according to reflection geometry, and have a large reflection angle (10.degree. to 40.degree.) to avoid reflected light going back to where it came from. As a result, a position where filter 116 is placed has a tolerance level in the order of micro-meters. Unfortunately, such a small tolerance in device manufacturing precision results in a low yield.
Therefore, waveguide devices having folding configurations are known to exist. The problem is, however, that a process for creating these devices with sufficient precision is not known.